Nitride semiconductors (Al, In, Ga)N have bandgap energies ranging from about 0.7 eV to 6.2 eV according to their composition ratios. Heterojunction structures composed of these materials have been widely applied to light emitting devices emitting light in the visible and ultra-violet region.
The conventional technology in this field has generally employed an n-i-p structure which is formed on a substrate by sequentially growing an n-type semiconductor doped with donors, an intentionally undoped i-type active layer, and a p-type semiconductor doped with acceptors.
There are several important requirements for a semiconductor-based light emitting device to have a high efficiency. However, it is essential that an electron provided from the n-type electrode and a hole provided from the p-type electrode are evenly diffused in the n-type semiconductor and the p-type semiconductor, respectively, and then recombined in the active layer between these layers by means of the optical transition.
Since holes in most nitride semiconductors have a lower mobility than electrons, it is important that the diffusivity of hole in the p-type nitride semiconductor be increased in order to improve the emission characteristic of a nitride semiconductor-based light emitting device. The diffusivity of hole in the p-type nitride semiconductor depends on several factors such as the concentration of holes, the effective mass of hole, and the concentration of impurities serving as scattering centers of the hole, etc.
Conventionally, to increase the diffusivity of hole in p-type nitride semiconductors, a method of increasing the hole concentration in the p-type nitride semiconductor has been used. GaN generally becomes a p-type semiconductor when it is doped with magnesium atoms and then annealed, thereby converting magnesium impurities into acceptors and releasing free holes. The maximum concentration of holes commonly obtained by this method is approximately 5×1017 cm−3.
On the other hand, an n-type GaN may be formed by doping silicon atoms serving as donors, and the concentration of free electrons routinely obtained by this method is approximately 1×1018 cm−3. Because there is a large difference between electron and hole concentrations usually achieved in the n- and p-type GaN semiconductors, respectively, a novel structure that increases the concentration and mobility of hole is needed to realize a light-emitting device having a higher efficiency than the current level.
The present inventors discovered that carrier diffusion and current spreading can effectively be achieved and used to enhance the emission efficiency of nitride semi-conductor-based light emitting devices consisting of an n-type semiconductor grown on the substrate followed sequentially by an i-type semiconductor and a p-type semi-conductor. This structure has the wurtzite lattice structure with the Ga face, and the spontaneous and piezoelectric polarizations in the n-i-p hetero-structure assist the accumulation of free electrons and holes at the n-i interface and the i-p interface, respectively. This effect gives rise to carrier diffusion and current spreading at the hetero-interfaces.